The present invention relates to a method of processing seismic data. Such a method may be used for marine seismic data or land seismic data and for two dimensional or three dimensional data.
In seismic exploration, seismic data are gathered by actuating one or more sources of seismic energy and recording at one or more locations reflected seismic energy so as to provide information about the reflective boundaries below the surface of the earth. For instance, in the case of a typical marine seismic exploration, a seismic source comprising one or more air guns or the like and a seismic streamer comprising hydrophones spaced regularly along a cable are towed behind an exploration vessel. The source is periodically actuated or fired and the reflected pressure variations detected by the hydrophones are recorded as individual traces. The data are therefore in the form of a large number of traces which are stored and subsequently processed using various techniques.
The traces are normally re-ordered or arranged into common mid-point gathers. The midpoint is the middle of a straight line extending from the source to the receiving transducer for the trace, so that all traces having the same mid point are grouped together and are sorted as a function of offset i.e. the distance between the source and the receiver. The travel times of the incident and reflected waves vary for different offsets, and this is corrected by means of normal move out (NMO) correction. Optionally, dip move out (DMO) correction is applied so as to compensate for non-horizontal reflective boundaries beneath the surface of the earth. The corrected traces are then "stacked" i.e. added together for each common mid-point gather, resulting in one stacked trace per common mid-point position.
For a three dimensional exploration giving information about a volume of the earth below the surface, the reflected seismic wave field may be considered as a five dimensional space having two spatial coordinates for the mid-point, one spatial coordinate for the offset, one spatial coordinate for the offset direction or azimuth, and a time coordinate. Instead of offset and azimuth coordinates, the offset may be defined in terms of spatial i.e. cartesian coordinates. For a two-dimensional exploration of a vertical slice through the earth, the mid-point and offset may each be defined by a single spatial coordinate so that three dimensional space is sufficient. The seismic wavefield is normally sampled at regular discrete time intervals and irregular discrete mid-points and offsets.
The reflected seismic wave field after NMO correction may comprise three parts. The first part depends on the mid-point position and varies only slowly with offset. A second part depends on the offset and varies only slowly with mid-point position. The third part comprises the rest of the seismic wavefield.
All three parts of the seismic wavefield contain useful information. The first part provides useful seismic information about the structure of the earth. The second part comprises offset-dependent or shot-generated noise whereas the third part contains all other types of noise. The second part includes the major types of noise in the seismic wavefield, such as noise from waves which propagate horizontally along the surface e.g. "ground roll" for land data.